The present disclosure relates to a device for compensation of wave induced distance variations on a drill string between a floating rig and a seabed-fixed installation.
When drilling offshore, it has become common to use floating units which at least during the actual drilling operation are fixedly attached to a fixed installation where the drill penetrates the seabed. For the purpose of compensating for wave induced distance changes, i.e. vertical changes in distance from the seabed to the floating drilling rig utilized, a so-called heave compensation is in continuous operations. However, such compensation will fall sooner or later, and it is thus desirable, and has eventually become a requirement, that there is an extra device to ensure that the equipment is not torn apart by the forces of waves if the primary heave compensation for any reason ceases to work.
WO publication 2011 074984 teaches a system for dealing with the above problem, comprising a trigger module for attachment to a tubing string in a heave-compensated, load-bearing unit disposed on a floating installation, wherein two or more hydraulic cylinder units form an extendable connection between the heave-compensated, load-bearing unit and a portion of the tubing string. The hydraulic cylinder unit must have fluid communication with an accumulator unit and there is a need for hydraulics and accumulator fluid pipes between the hydraulic cylinder unit and a gas reservoir in a suitable manner. A disadvantage with this solution is that the accumulator is not arranged on the heave compensated unit. This gives a higher risk of a safety failure, since the fluid communication pipe is exposed to external elements that might deform it or tear it apart.
The installation time for the described heave compensated unit can be extensive and there is a risk of erroneous installation due to the number of components of the system. Any errors might lead to a stop in the floating installation in question and damage to the equipment, There is thus still a need for a simple, robust, compact and reliable device that can ensure that the need for wave induced distance variations between a floating rig and a seabed-fixed installation.